Journal of Non-Crystalline Solids 385 (2014) 55–74
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids
journal homepage: www.elsevier.com/ locate/ jnoncrysol
An overview on silica aerogels synthesis and different mechanical
reinforcing strategies
Hajar Maleki ⁎, Luisa Durães, António Portugal
CIEPQPF, Department of Chemical Engineering, Faculty of Sciences and Technology, University of Coimbra, Rua Sílvio Lima, 3030-790 Coimbra, Portugal
a r t i c l e
i n f o
Article history:
Received 2 May 2013
Received in revised form 2 October 2013
Available online 22 November 2013
Keywords:
Silica aerogels;
Sol–gel;
Mechanical reinforcement;
Hybrid materials
a b s t r a c t
Silica aerogels are lightweight and highly porous materials, with a three-dimensional network of silica particles,
which are obtained by extracting the liquid phase of silica gels under supercritical conditions. Due to their outstanding characteristics, such as extremely low thermal conductivity, low density, high porosity and high specific surface
area, they have found excellent potential application for thermal insulation systems in aeronautical/aerospace and
earthly domains, for environment clean up and protection, heat storage devices, transparent windows systems, thickening agents in paints, etc. However, native silica aerogels are fragile and sensitive at relatively
low stresses, which limit their application. More durable aerogels, with higher strength and stiffness, can be
obtained by proper selection of the silane precursors, and constructing the silica inorganic networks by
compounding them with different organic polymers or different fiber networks. Recent studies showed
that adding flexible organic polymers to the hydroxyl groups on the silica gel surface would be an effective
mechanical reinforcing method of silica aerogels. More versatile polymer reinforcement approach can be
readily achieved if proper functional groups are introduced on the surface of silica aerogels and then copolymerized with appropriate organic monomers. The mechanical reinforced silica aerogels, with their
very open texture, can be an outstanding thermal insulator material for different industrial and aerospace
applications.
This paper presents a review of the literature on the methods for mechanical reinforcing of silica aerogels and discusses the recent achievements in improving the strength and elastic response of native silica aerogels along with
cost effectiveness of each methodology.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Silica aerogels are materials with unique properties such as high specific surface area (500–1200 m2.g−1), high porosity (80–99.8%), low density (∼0.003–0.5 g.cm−3), low thermal conductivity (0.005–0.1 W/(mK)),
ultra low dielectric constant (k = 1.0–2.0) and low index of refraction
(∼1.05) [1,2]. Due to their such unusual characteristics, much attention
has been given to silica aerogels in recent years for their use in several
technological applications including Cherenkov radiators in particle physics experiments [3] and thermal insulation materials for skylights and
windows [4]. Silica aerogels have also been used for making heat storage
devices used in window defrosting and as acoustic barrier materials [5].
Other aerogels have been demonstrated as battery electrodes [6], catalyst
supports [7], and oxygen and humidity sensors [8] and adsorbents for environmental clean-up [9] due to their large internal surface areas and facile changing of their surface chemistry. The low values of thermal
conductivity and the very low density make silica aerogels attractive
⁎ Corresponding author. Tel.: +351 910 156 989; fax: +351 239798703.
E-mail address: [email protected] (H. Maleki).
0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jnoncrysol.2013.10.017
materials for a number of aerospace applications. One example involves insulation around the battery packs in the Mars Sojourner
Rover to protect its electronic units [10]. More robust and flexible
aerogels are being considered to insulate extra-vehicular activity
(EVA) suits for future manned missions to Mars [11]. Aerogel composites are the only materials that come close to meet the requirements for EVA suit insulation [12]. Robust aerogel composites are
also considered as insulation materials of inflatable decelerators
for entry, descent, and landing (EDL) applications for future space
missions on Mars [13].
It should be noted that aerogel applications in space are not all limited to thermal insulation. Indeed, silica aerogels can also be applied to
collect aerosol particles [14], to protect space mirrors or to design tank
baffles [15,16]. However, these applications of silica aerogels have been
restricted because of their extreme fragility and poor mechanical
properties and hygroscopic nature [17]. Therefore, with the purpose
of expanding the application range of aerogels, while fully retaining
their outstanding properties, mechanically more robust aerogels are
needed.
Different methods have been explored to improve the mechanical
properties of silica aerogels such as structural reinforcement using
flexible silica precursors in silica gel backbone [18–21], conformal
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H. Maleki et al. / Journal of Non-Crystalline Solids 385 (2014) 55–74
coating of silica backbone via surface cross-linking with a polymer
[22–26], and dispersing carbon nanofibers in the initial sol of silica
aerogel [27]. In principle, to improve flexibility or elastic recovery
in silica aerogels, it is required either to include organic linking
groups in the underlying silica structure or to cross-link the skeletal
gel framework through surface silanol groups by reacting them with
monomers/polymers.
Since the silica aerogels consist of silica particles that are connected to
each other via only Si\O\Si stiff bonds, the compounding of the silica
aerogel with polymer leads to an increase of the connection points between the silica particles, with the formation of extra strong \(C\C\)
covalent bonds between these particles [28]. Therefore, such methodology leads to an increase of the strength of the reinforced silica aerogels over
native silica aerogels [29]. Thus, the objective of such compounding
with organic polymers is to provide an aerogel with good compressive
strength to be able to adapt to the design of components and to adsorb
the energy involved in shock compressions [14,15,30,31]. However,
manufactured aerogels are not strong enough to be reshaped and
must be casted to the final forms during synthesis and processing
[32]. But, it has been proven that elasticity/flexibility can be significantly
enhanced in non cross-linked aerogels by altering the chemical nature
of the silica backbone. For example, for TMOS/BTMSH-derived aerogel,
with densities bellow 0.06 g.cm−3, it was possible to bend by 50° the
material without breaking it [33]. Additionally, Kramer et al. [34] demonstrated that an addition of up to 20% (w/w) poly(dimethylsiloxane)
in (TEOS)-derived aerogels resulted in rubbery behavior with up to
30% recoverable compressive strain. Shea and Loy [35] have developed
hybrid aerogels from bridged polysilsesquioxanes, using building blocks
comprised of organic bridging groups attached to two or more
trialkoxysilyl groups via nonhydrolyzable carbon\silicon bonds. All of
these methods proved to be different ways of tuning mechanical properties to the application requirements.
As it is indicated by Fricke [36] and Pekala et al. [37], due to the
tradeoff between mechanical properties (e.g. Young's modulus or
maximum strength) of silica aerogels with their density, the most
straightforward methods of mechanical reinforcing of silica aerogels
result in increasing the density and therefore increasing the thermal
conductivity [38–40]. This is caused by the increase of the total
amount of material used for the production of the gel matrix, due
to the need of increasing the total number of connection points
within the silica aerogel. The most recent achievements involve
the preparation of aerogels with approximately three order of magnitude improvement in their maximum compression strength at
break with only doubling the density and thermal conductivity
[29]. Therefore, the purpose of this review is to provide an overview
about versatile methods to impart higher strength or stiffness to silica aerogel monoliths with minimum increase of their density and
thermal conductivity.
General physical and chemical issues involved in the synthesis of
silica gels were explained in books [41,42]. Additionally, many reviews on aerogels, with particular focus on silica aerogels, have already been published [1,43–47], which give a more specific and
complete description about the aerogels' processing, properties and
applications. For the purpose of strengthening the mechanical properties of silica aerogels, different possible strategies of surface chemistry
modification of aerogels, followed by compounding their surfaces
with appropriate organic polymers, namely epoxide [48,49], polyurea
[50], polyurethane [33,51], polyacrylonitrile [22], and polystyrene
[52,53], are reviewed by Leventis et al. [26,54], and more recently by
Meador et al. [29,32].
The present review is composed by two parts: firstly, a general history
of silica aerogels with chemistry beyond their methods of synthesis and
the principles of the drying techniques is given; secondly, an extensive
study of very recent published works about the development of silica
aerogels with improved mechanical properties is presented, with particular emphasis on recent advanced methods for preparing class II hybrid
polymer–silica aerogels. The main objective is to review the possible
physical and chemical existing methodologies to improve the mechanical
properties of silica aerogels, which were not under scope of studying by
the previous authors. Additionally, the present review surveys the use
of different kinds of nanofiber networks, including polymeric [55–57], ceramic [58,59] and carbon nanofibers [58], in order to develop the free
standing silica with improvements on skeletal maintenance of native silica or even polymer reinforced aerogels. Finally, the future trends for development of flexible and bendable hybrid thin sheets of silica aerogels
are addressed, being these materials suitable for folding or wrapping
around different space assemblages, such as around the electronics and
battery packs in the Mars Sojourner Rover [10]. Moreover, the development of smart silica aerogels using shape memory polymers [60] is also
mentioned as a major future development and is described in the last
part of the present review.
2. Sol–gel chemistry
Generally, a nanostructured solid network of silica is formed as a result of a hydrolysis and condensation process of the silica precursor
molecules, in which siloxane bridges (Si\O\Si) are formed. Such reactions are equivalent to a polymerization process in organic chemistry,
where bonds between the carbon atoms of organic precursors lead to
linear chains or branched (cross-linked) structures [61].
The manufacturing process to form silica aerogels comprises two
steps: the formation of a wet gel by sol–gel chemistry, and the drying of
the wet gel. Originally, silica wet gels were made by Kistler in the early
1930s [62,63] through the condensation of sodium silicate (also termed
as waterglass). However, the reaction formed salts within the gel that
needed to be removed by many long and laborious repetitive washings
steps [64]. In following years, Teichner's group extended this approach
to prepare optically transparent monoliths using tetraalkoxysilanes
(Si(OR)4) as the silica source [65]. The key of this process was to use an
alcohol (e.g. methanol or ethanol), thus, the need for the tedious water
to alcohol solvent exchange was eliminated [65–69]. This reduced the
time required to make a final dried aerogel to approximately one day,
which was a drastic reduction relatively to Kistler's original method. Presently, with the further developments of the sol–gel process, different
alkoxysilane derivatives are used all together and with different solvents,
which are needed for the homogenisation of the mixture and control of
concentrations. Although, when switching from a protic to an aprotic (hydrocarbon) medium, alcohols are ideal intermediary solvents, as their bifunctional nature (polar/non-polar) promotes miscibility of water and the
organic phase. But, surprisingly, the choice of the alcohol has a tremendous effect on pore structure and therefore also on final material
properties [70–72]. The indication of different gelation solvents and related methodologies applied by different investigators, along with the different properties of the resulting aerogels, is reviewed by Soleimani et al. and
Siouffi [47,73].
Due to the rapid development of sol–gel chemistry over the last few
decades, the majority of silica aerogels are prepared today using silicon
alkoxides as precursors [74–80]. The most common of the silicon alkoxides are the tetramethylorthosilicate (TMOS, Si(OCH3)4) and tetraethylorthosilicate (TEOS, Si(OCH2CH3)4) [42], with common chemical
formula of Si(OR)4, that lead to the aerogels called “Silica”. Many
other alkoxides [81], containing various organic functional groups
linked to silicon, can be used to give different properties to the gel.
They have a general formula R′XSi(OR)4-X (1 ≤ X ≤ 3), being mono, di
and trifunctional organo silanes, which lead to the aerogels named as
“orgonosilsesquioxanes” [78]. Other common organo silica precursors
with chemical formula of (OR)3SiR′Si(OR)3 in which R′ is an alkyl, aryl
or alkenyl bridge group between two elements of silica, lead to the
“bridged organosilsesquioxane” products [19,35,82,83]. Alkoxidebased sol–gel synthesis avoids the formation of undesirable salt byproducts, and allows a much greater degree of control of the final
H. Maleki et al. / Journal of Non-Crystalline Solids 385 (2014) 55–74
products. The chemical equations for the synthesis of a silica gel from
TEOS are as follows:
Hydrolysis
Condensation:
57
involve soaking the gel in an alcohol/water mixture of equal proportions
to the original sol at a pH of 8–9 (ammonia) [90–92]. It has also been
demonstrated that simply performing a thermal aging of the wet gel
in water can be a key factor to decrease the gel microporosity before
drying [42].
Generally, two different mechanisms might operate during aging that
affect the structure and properties of a gel: (a) neck growth, from
reprecipitation of silica dissolved from particle surface onto necks between secondary particles (Fig. 1(i)); and (b) dissolution of smaller particles and precipitation onto larger ones (Ostwald ripening mechanism)
[89]. These two mechanisms will operate at different rates, but simultaneously, as illustrated in Fig. 1(ii). In addition, the particle clusters are
brought in contact by Brownian motion and react with each other, increasing the number of siloxane bridges and reinforcing the silica network in neck regions [93]. The most effective and applied methods to
overcome the weak mechanical properties of silica aerogels are reviewed
in Section 4 of this manuscript.
3. Drying techniques
The above reactions are typically performed in methanol or ethanol
[66,67]. The final density of the aerogel is dependent on the concentration of the silicon alkoxide monomers in the solvent.
The significant change from the liquid to the solid stage is termed the
sol–gel transition. When a sol reaches the gel point, it is often assumed
that the hydrolysis and condensation reactions of the silicon alkoxide
reactant are complete. During this event, initially, the primary particles
are formed, then they aggregate into secondary particles, and finally link
together in a pearl necklace morphology [84]. The diagram presented in
Fig. 1 shows the 3D network of porous silica, which is constructed by
primary and secondary silica particles.
The kinetics of the sol–gel reactions is slow at room temperature and
often requires several days to reach completion. For this reason, acid or
base catalysts are added to the system [85,86]. The amount and type of
the used catalysts play key roles in the microstructural, physical and optical properties of the final aerogel product. Acid catalysts can be any
protic acid, such as HCl [18]. Basic catalysis usually uses ammonia, or
ammonia buffered with ammonium fluoride [42].
Since the silica aerogels comprise highly open structures in which
the secondary particles of silica are connected to each other with only
few siloxane bonds, the structure of native aerogels is too fragile to be
handled. One elegant but time consuming method to strengthen the
solid skeleton of a silica gel is to enlarge the connection point between
the secondary particles with more siloxane bonds via an “aging process”
[87–89]. Common aging procedures for base catalyzed gels typically
The final, and most critical, process in the synthesis of silica aerogels
is the drying step. This is when the liquid within the gel is removed leaving only the linked silica network. Three main routes are commonly
used for drying: (1) freeze-drying, in which the solvent inside of pores
needs to cross the liquid–solid then the solid–gas equilibrium curve;
(2) evaporation, which implies the crossing of the liquid–gas equilibrium curve of the solvent; (3) supercritical fluids drying (SFD), in which
the supercritical condition is reached without crossing the equilibrium
curve of the solvent [61].
Generally, in the freeze drying technique, the solvent in the pores is
frozen and then sublimed under vacuum. The material obtained in this
way is called a “cryogel”. However, due to crystallization of the solvent
within the pores, this process leads to cracked or even powder-like silica
products with very large pores [42]. But this problem can be attenuated
by using solvents with low expansion coefficient and high sublimation
pressure, and also by using rapid freezing in liquid nitrogen at cooling
rates over 10 Ks−1 [94,95].
The capillary pressure, Pc (Pa), that develops during evaporative drying, causes the shrinkage of the gels. This pressure can be calculated by
[42],
−γ 1v
Pc ¼ r p −δ
ð1Þ
Fig. 1. Typical SEM image of Si aerogels with schematic representation of primary and secondary silica particles (left); (i) neck growth mechanism of secondary silica particles
and (ii) relative aging rate as a function of time for two mechanisms (a, b) (right, adapted from [93] and [84]).
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H. Maleki et al. / Journal of Non-Crystalline Solids 385 (2014) 55–74
γ1ν is the surface tension of the pore liquid (N/m), rp is the pore radius
(m); δ is the thickness (m) of a surface adsorbed layer.
Evaporation without specific surface treatments [96–99] usually
results in “dense” (e.g., N0.25 g.cm−3 [98]) and cracked materials, the
so-called “xerogels”. As explained by Phalippou et al. [100], the densification during evaporation comes from condensation of the remaining
reactive silica species. When the silica wet gel is subjected to the capillary pressure, the initially far distance surface hydroxyl/alkoxy groups
come close enough to each other to react and generate new siloxane
bonds, leading to the irreversible shrinkage due to the inherent flexibility of silica chains. In addition, often the pore structures of xerogels collapse when compared to those of aerogels with the same composition
[79,99,101–104]. Due to the different existing pore sizes within the
gel, a high capillary pressure gradient develops inside the porous structure during the drying, which leads to mechanical damage. The capillary
tension in evaporative drying may reach 100–200 MPa [47], and since
the siloxane bonds within the aerogel monolith are not strong enough
to withstand the capillary pressure then it results in shrinked and
cracked xerogels [105].
One of the ways to overcome this problem is by strengthening the gel
to stand the capillary stresses. This can be achieved by replacing some of
siloxane (Si\O\Si) bonds with flexible and non-hydrolysable organic
bonds (Si\R), through the use of organosilanes as precursors to produce
the aerogel network [20,106,107]. The organic group will allow the
aerogel to spring back to its original wet gel size without resulting in
any crack within the gel. Other methods involve changing the capillary
force experienced by the network through surface modification of the
silica with alkyl groups, and providing a surface with lack of Si\OH
groups [108]. Here, the general goal for the surface treatment is to produce a more hydrophobic surface by reacting the surface hydroxyl
groups with hydrophobic reagents, such as [(CH3)3-Si-OR] [109,110], or
hexamethyldisilazane [111,112]. On the contrary to the surface rich
Si\OH gels, during the evaporation of the solvent in the hydrophobically
surface treated gels, the alkyl groups repel one another originating the
referred spring back of the aerogel [70,106,113]. The other methods to
overcome the induced capillary pressures involve using low surface tension solvents [114], or adding additives to control the drying process
[68]. Evaporation of a low surface tension solvent from the silica wet
gel network reduces the capillary pressure when compared to the evaporation of an alcohol, since there is a direct relation between the surface
tension and capillary pressure [115] according to Eq. (1).
In the supercritical fluid drying method, the liquid in the pores is removed above its critical temperature (Tcr) and pressure (Pcr), i.e. in supercritical state. In this condition, there are no liquid–vapor interfaces
and, thus, no capillary pressure gradients arise. The supercritical fluid
drying method can be performed: (1) with organic solvents in their supercritical state (generally with the synthesis alcohols and, consequently, near 260°C if ethanol or methanol is used), which is called HOT
process [42]; or (2) by extracting the synthesis solvent with supercritical CO2 at a temperature slightly above the critical temperature of CO2
(~31°C), which is called COLD process. In the COLD process, the liquid
part of the wet gels has to be exchanged with CO2, either in the liquid
state [116], or directly in the supercritical state [117]. This process produces monolithic silica aerogels of rather large dimensions (for the typical cylindrical samples, the diameter is about 1–2 cm and length 5–
6 cm), but the dimensions of aerogel are obviously scaled by the sizes
of the supercritical drying autoclave [28]. The lower temperature in
the drying step and non-flammability of CO2 make the COLD process
safer and cheaper than the HOT process. However, the development of
the evaporative drying process to dry the silica wet gels by properly
controlling the critical drying parameters (T, P, evaporation area and
times) to achieve a xerogel material with final properties comparable
with their aerogel counterpart, i.e. an aerogel-like material, would be a
tremendous potential for scaling-up and cost reduction of such a material [99,118,119]. However, from an economical and large scale production point of view, there are also other items which can be significant in
cost reduction of such materials [119]. Especially the cost of the starting
silica precursors (normally, sodium silicate is cheaper than silicon alkoxides) and the several processing steps (e.g. the washing, the solvent
exchange, and surface treatment) influence the global cost of the final
aerogels or xerogels [64,96].
Fig. 2 illustrates the complete aerogel synthesis process with its
detailed steps, as explained before. Generally, the silica aerogels
are made in two main steps: (1) the sol–gel process (the preparation of an alcogel) and (2) supercritical drying. Initially, the alcogel
is prepared by promoting the hydrolysis and condensation reactions
of silica precursors in the solution with solvent/water/catalysts,
based on sol–gel chemistry. During this step, the primary silica particles will be created, and, then, during gelation, the primary particles coalesce and link to each other to form the secondary silica
particles (see Fig. 1), resulting in a rigid three-dimensional network
of silica. Then, the prepared silica wet gels can be subjected to different post-gelation treatments before drying, such as solvent exchange, washing and aging. In the drying step, the silica aerogel
monolith can be obtained when the solvent inside the pores experience its supercritical condition and is released in this state without
introducing any damage in the solid part.
4. Mechanical reinforcement of silica aerogels
Silica aerogels are cellular solids with a pearl-necklace-like skeletal
network [84]. As indicated in Fig. 1, the weak points of such structure
are the interparticle neck regions [120,121]. Although the native silica
aerogels are sufficiently strong to be handled, their mechanical
strength still is not adequate for them to endure and remain monolithic
in some practical applications [54]. In fact, their intrinsic fragility that
leads to low mechanical strength, imposes severe constrains on different potential load bearing applications of silica aerogels. Several techniques have been reported in the literature [28,29] to reinforce silica
aerogels' mechanical properties. The aim of such techniques is to develop aerogels of low density that can be easily deformed and show a
capability to absorb shock energy during bending and compression
[14,30].
As stated before, the aging of wet gels leads to mechanically stronger
inorganic networks [121,122], by increasing the strength of the final silica aerogels through dissolution and reprecipitation of silica at the surface of interparticle necks [123]. With this process, an improvement of
approximately a factor of two in the modulus of elasticity will be
achieved. But clearly, at the end, the reinforcement agent is still silica
and remains a brittle material with low tensile strength.
Hybridization of silica aerogels [34,124,125] can be an alternative solution for strengthening purposes, by promoting the co-gelation of the
silicon alkoxide with hybrid precursors such as poly(dimethylsiloxane)
(PDMS). Gels obtained in this way are termed “ORMOSIL” (ORganically
MOdified SILica) hybrids. They have a more rubber-like flexibility. With
20 wt.% PDMS, they can be elastically compressed to 30% (by volume)
with no damage [34].
Compounding of the inorganic network of aerogels with different
polymeric systems has been performed by several chemical procedures. This method leads to a dramatic increase of tensile strength
and robustness of the aerogels [26]. Moreover, incorporation of various fibrous supporting materials, such as polymeric fibers [55,57],
carbon nanofibers [26], and fiberglass [126], into the aerogel systems, was also found to be quite effective in improving the mechanical properties of aerogels. Fiber matrix can support the aerogel and
decrease the bulk size of aerogel within aerogel-fiber matrix composite [127]. So far, composites of fiber and silica aerogels have
been produced with various methods in order to fortify the structure
of silica aerogels.
The next sections of the present review paper will address all of
above mentioned mechanical reinforcement strategies of the silica
H. Maleki et al. / Journal of Non-Crystalline Solids 385 (2014) 55–74
59
Fig. 2. Schematic representation of typical sol–gel synthesis procedure.
aerogels, and are a rather complete and updated literature survey on the
subject.
4.1. Structural reinforcement of silica aerogels
Generally, the trifunctional organosilane compounds of the type
RSiX3 (where, R = alkyl, aryl or vinyl groups, X = Cl or alkoxy groups)
produce flexible aerogels with reduced overall bonding and good
hydrophobicity [80,128], since one of the ends of Si atom contains a
non-hydrolysable R group. Due to the presence of this organic group, R,
attached to the silica polymer chains, the inter-chain bonding is reduced
resulting in an elastic and flexible three-dimensional matrix [19].
Kanamori et al. have shown that MTMS-derived gels can have reversible
deformation upon compression [129]; Fig. 3 shows schematically the
molecular structure of MTMS-derived silica aerogels. Rao et al. [19,107]
studied the elasticity of these aerogels in terms of the Young's modulus
Fig. 3. Three dimensional network of MTMS-derived aerogels with its detailed molecular structure.
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H. Maleki et al. / Journal of Non-Crystalline Solids 385 (2014) 55–74
(γ), using uniaxial compression measurements. Aerogels with high elasticity, flexibility and with Young's modulus as low as 1.094 × 104 N.m−2
could be obtained. They attributed the easiness of hydrolysis and condensation reactions to the length of methyl and methoxy groups within
the silica precursor, as these are the smallest among all alkyl and alkoxy
groups.
The MTES (H3C-Si-(OC2H5)3) precursor has also been used as a
trifunctional organosilane compound to synthesize superhydrophobic
and flexible aerogels [20,130,131]. In this case, each monomer of the
MTES has one non-hydrolysable methyl group (CH3), as in MTMS, and
three hydrolysable ethoxy groups (OC2H5). Therefore, only hydrolysable ethoxy groups are responsible for the matrix formation. As the condensation and hence the polymerization progresses, the number of
hydrophobic Si\CH3 groups increases compared to the number of hydrophilic Si\OH groups, leading to an inorganic–organic hybrid silica
network which is superhydrophobic and highly flexible and can recover
or springs back after compression [20,107]. For the synthesis of such trifunctional organosilica based aerogels, Rao's research team [20] applied
both the single-stage (base catalyzed) and two-stage (acid–base catalyzed) sol–gel processes. They have found that the two-stage process,
due to step wise acid catalyzed hydrolysis and then base catalyzed condensation, leads to the built up of a systematic, complete and continuous network of silica, and thus, leads to highly flexible aerogels that
can be compressed to as high as ~60% by volume [20,132]. The singlestage process leads to less flexible aerogels (b 5% compression by volume), with non-continuous silica network, as a result of competitive hydrolysis and condensation occurring during the sol–gel process [20].
Apart from the synthesis methodology, the synthesis parameters,
such as the rate of hydrolysis and condensation reactions, aging periods
and dilution factor of silica precursors, have been found to have influence on the obtained mechanical and rheological properties of the trifunctionalized organosilica derived aerogels. For example, the effect of
the precursor concentration on the elastic properties was examined,
for the cases of MTMS and MTES, by varying the MeOH/MTMS and
MeOH/MTES molar ratios (S) [19,20]. The used molar ratios in the case
of MTES-derived aerogels were S = 6.45, 12.96 and 19.35, and in the
case of MTMS-derived aerogels were S = 21, 28 and 35. It was found
that the density of the aerogel decreased with increasing S ratios
[19,20], and the Young's modulus (γ) of aerogels scaled with the bulk
density in the same manner. For MeOH/MTMS ratios within the range
from 14 to 35, the volume shrinkage and the density of the corresponding aerogels decreased from 28 to 7% and from 100 to 40 kg/m3, respectively. The higher dilution of the MTES precursor (MeOH/MTES=19.35)
also resulted in the increase of the gelation time and in lower degree of
polymerization of silanols and, thus, increased flexibility. The schematic
image in Fig. 4(a, b) shows this molar ratio effect on the silica aerogel
network. As it is clear, increasing the S molar ratio (MeOH/MTES =
19.35), the distance between the reacting silica monomers and
oligomers increases. Therefore, higher gelation time and less polymerization degree are observed. In addition, for higher dilution, the silica
chains are quite separated from each other and large empty spaces or
pores are formed in the aerogel network, leading to flexible aerogels.
On the contrary, at lower dilution of the MTES precursor (MeOH/
MTES = 6.45), due to the less separation distance between the silica
monomers and oligomer chains in the sol, the reacting species can
react easily, leading to extensive polymerization in three dimensions,
which results in dense and rigid polysilsesquioxane structures [19,20].
Fig. 5(a, b) shows the change in length versus the applied load for
both MTMS and MTES-derived aerogels synthesized with different dilutions of the silica precursor. As shown in this figure and based on the
above arguments, both types of aerogels, for higher dilution, show
higher changes in their lengths when compared with the aerogels that
were prepared with lower dilution (in the same applied loads). This observation proves the claim that higher dilution of organosilane precursors results in aerogel with lower Young's modulus and, therefore,
higher flexibility. Fig. 6(a–c) shows the maximum bending possibility
for MTES-derived aerogels with different dilutions of the silica precursor. The above arguments can be further confirmed by these photographies. As it can be seen, the lowest dilution of organosilane leads to less
flexibility; on the contrary, the aerogels with the highest S ratio show
the highest flexibility. Further bending of these samples resulted in
crack formation within the structure. Because of this new property,
i.e. flexibility, the aerogel can be bent to any shape and acts as a
good shock absorber as well [20].
The elastic limit of the prepared flexible MTES-derived aerogel
could be determined via a uniaxial compression test by applying different loads. Fig. 7 shows the mechanical behavior of the MTESderived aerogel with MeOH/MTES molar ratio (S) of 19.35. As
shown in this figure, the decrease in the length of the aerogel is almost linear with the applied load (path from point O to point A). At
point A, there is a sudden decrease in the compression, indicating
no further compression. Within the area under the curve, the aerogel
resists the deforming forces by bringing into play internal restoring
forces and recovers its original length when the deforming forces
are withdrawn [20].
Durães et al. [79,80,99] also studied the effect of the S molar ratio and
drying parameters on resulted properties of silica aerogels/xerogels
with the samples containing 100% MTMS, 100% ETMS and for the
aerogels/xerogels with 25% ETMS and 75% MTMS. It was concluded
that higher molar ratios of silica precursor/solvent (S) resulted in less
extension of the cross-linking and, consequently, lower density of the
materials and higher flexibility. It was also found that with increasing
the size of R group from methyl (MTMS) to ethyl (ETMS) group, more
linear structures of secondary silica particles would be created due to
the increase of steric hindrance in the medium and the consequent decrease of the condensation rate [80].
Fig. 4. Degree of polymerization of silanols exhibiting (a) flexible structure and (b) rigid structure. Reprinted from Ref. [20], Copyright (2009), with permission from Elsevier.
H. Maleki et al. / Journal of Non-Crystalline Solids 385 (2014) 55–74
61
Fig. 5. a) Change in length as a function of mass applied for the polysilsesquioxane aerogels prepared with various MeOH/MTMS molar ratios (21, 28 and 35). b) Change in length (ΔL) as a
function of the mass (m) applied on the MTES-derived polysilsesquioxane aerogels samples prepared with S = 6.45, S = 12.9, and S = 19.35. Reprinted from Refs. [19,20], Copyright (2006)
(2009), with permission from Elsevier.
A limited selection of the monofunctional and difunctional
organosilane compounds of the type RSiX3, R2SiX2 as well as X3Si-RSiX3 bis silane precursors, in which R is an alkyl or aryl group, is provided in Fig. 8 [83,133–136]. In this figure, different organosilica precursors
are separated under different categories and, from them; various
aerogels with different material properties can be prepared. Some of
these silica precursors contain special organic functionalities that care
useful for surface decoration or surface treatment of the aerogel for application of one's needs. To a certain extent, the inorganic–organic (hybrid) materials (ORMOSIL) that can be obtained from these precursors
combine the most important properties of their constituents, like high
transparency (glasslike), low processing temperatures (polymer-like),
sufficient thermal stability (silicone-like), and are easily prepared because of a unique availability of the respective precursors.
Besides the simple metal or silicon alkoxide precursors (for example,
TMOS) that, after hydrolysis and condensation (see sol–gel reaction
mechanism at Section 2), lead to the formation of an inorganic oxidic
network with only siloxane (Si\O\Si) bonds, the organo (alkoxy)silanes, depending on what type of functionalities they have, can be
used to incorporate polymerizable organic substituents, such as epoxy,
vinyl, or methacryloxy groups, into the final aerogel product [29]. The
Si\C bonds in such molecules are stable under the mild conditions of
sol–gel processing [137].
In addition to hybrid organo (alkoxy) silane precursors, the crosslinking of the silica backbone with more flexible and elastic alkyl
bridged bis-silane precursors can improve the strength of the aerogels
systematically. For example, as reported by Meador et al. [21], a non-
cross-linked aerogel made from a combination of TMOS and BTSPD
showed complete recovery after 25% compression, with an approximate
modulus of 4 MPa and densities of 0.2 g.cm−3. A high concentration of
the disulphide in the formulation yielded lower density monoliths,
which recovered completely from as much as 75% compression, as
shown in Fig. 9(a). The inset of Fig. 9(b) shows the same sample before,
during and after compression with finger pressure. This sample recovered 98% of its original length during the first ten minutes after compression and, after thirty minutes, the recovery was 99.6%.
Similar results have been reported by Meador et al. [49,138], for
aerogels synthesized by replacing up to 40% of TEOS or TMOS in the silica backbone with the alkyl linked BTMSH. As can be seen from Fig. 10,
the BTMSH reduces stiffness in the silica backbone by replacing some of
the more rigid Si\O\Si bonds with flexible hexyl links. The resulting
aerogels presented a modulus up to 23MPa and almost complete recovery from a state of 25% compressive strain. The evidence of increasing
mechanical performance using hybrid organic–inorganic bridged
alkoxysilanes is consistent with the work of Loy, Shea and co-workers
[35,83,139], which have examined a wide range of precursors to synthesize bridged silsesquioxanes. Typically, bridge bis-silane precursors
allow the pore size control that is directly related to the nature and
length of the bridge. Best results for controlling the porosity were obtained using a stiffer structure such as an arylene chain. More flexible
bridges such as alkyl chains lead to more compliant aerogels but tended
to shrink more, reducing porosity.
More recently, Randall et al. [29] examined the relative merit of
three types of bis-silane precursors with different lengths of the alkyl
Fig. 6. Maximum possible bending of the MTES-derived aerogels prepared with a) S = 6.45, b) S = 12.96, and c) S = 19.35. Reprinted from Ref. [20], Copyright (2009), with permission
from Elsevier.
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Fig. 7. Compression of an MTES-derived aerogel sample as function of the applied compression load. Reprinted from Ref. [20], Copyright (2009), with permission from Elsevier.
groups on epoxy reinforced silica aerogels. As shown in Fig. 11, this research team applied four types of bridged alkyl linked bis-silanes and a
difunctional silica precursor to which the extended chemical structures
are shown in Fig. 8. In the case of co-gelation of bridged bis-silane precursors with TEOS, the flexible alkyl links/bridges between the secondary silica particles, depending on the size of the bridge, can confer
flexibility to the network. All of these precursors improved the elastic
recovery of the aerogels with an amount as small as 15% included in silica backbone. Randall et al. also reduced the number of Si\O\Si bonds
by using a difunctional silane, DMDES, in order to improve the elastic response by reducing the stiffness of the silica network. However, additional incorporation of bis-silane precursors (up to 45% of any bissilane) reduces the compressive modulus. Authors indicated that the
hexyl linked bis-silane (BTMSH) is the most effective among the bissilanes as compared with others in their work. Only BTMSH produced
aerogels with high mass yields, indicating complete hydrolysis and condensation under the reaction conditions they studied.
Nearly similar results have been reported by Vivod et al. [140], in
which the incorporation of the hexane linked precursor (BTMSH) in
the silica backbone was found to enhance the flexibility and the strength
of the aerogels. The wet gel could be bent easily without any fracturing
and this can improve the manufacturability of BTMSH-derived aerogels.
This sample also displayed great recovery at higher Si concentrations
after compression to 25% strain.
4.2. Polymer reinforced silica aerogels
It has been proved that compounding of silica backbone with polymer is an effective way to increase mechanical strength by as much as
three orders of magnitude while only doubling the density over those
of native or non-reinforced aerogels [32,119]. Compounding the silica
network with polymer can be achieved by different type of interfacial interaction of secondary silica particles (inorganic parts) with appropriate
functionality on the organic polymers. Depending on the chemical relationship between the polymer and the surrounding silica network, polymer/sol–gel composites are placed into two categories: Class I hybrid
composite aerogels and Class II hybrid composite aerogels [141–143].
4.2.1. Class I hybrid composite aerogels
The composites that are formed as a result of weak interactions, like
van der Waals forces, electrostatic forces, or hydrogen bonding, between the organic and inorganic phases are in the category of this
class of hybrid materials [144,145]. In this type of composite materials,
organic molecules, pre polymers or even polymers are embedded in the
inorganic matrix being totally independent from each other. These
materials are synthesized by carrying out a number of different routes,
e.g. the formation of the inorganic network through hydrolysis and
condensation of the silica precursors, in the presence of the organic
compound or by polymerizing organic monomers in the porous inorganic host. The most prominent examples representing class I are organic dyes or biomolecules incorporated in porous sol–gel matrixes
[146,147]. The guest molecules are physically dissolved together with
the precursors of the inorganic host (e.g., TEOS or TMOS) or introduced
to the sol state, and become entrapped in the gel that results from condensation and drying of the mixture.
Other examples of this group of hybrids are provided by composites
formed upon incorporation of different polymers, such as poly(ethylene
oxide) [148,149], derivatives of nylon 6 [150] and poly(ethyl acrylate)
[151], into SiO2 matrices synthesized by the sol–gel method. Here, in
the composites whose organic components have polar groups, the formation of hydrogen bonds between the components of the system
[152] is important to understand the nature of the synthesized materials. The hydroxyl groups in the repeating units of the polymer, like
ethylene glycol oligomers [149] or poly(vinyl alcohol) [153,154], are expected to produce strong secondary interactions with the residual
silanol groups generated from acid-catalyzed hydrolysis and polycondensations of SiO2 matrix.
Although the resulted hybrid gels possess proper transparency and
thermal stability without any phase separation between the two components, there is no particular example from this family of composite with
the purpose of mechanical reinforcing of native silica aerogels. Indeed,
this class of hybrid materials is traditionally used to improve the very
poor mechanical properties of organic polymers such as polysiloxanes
polymer (PDMS) with incorporation of an inorganic filler of SiO2
[155,156].
Since post-gelation washing is often required in the processing of
the silica aerogels, class I hybrid silica composite aerogels are rarely
studied. This is due to the existence of weak interfacial bonds between
the two phases, and consequent easy leaching of the polymer from
the pores of silica by the extensive solvent exchange, either during the
post-gelation washing steps or solvent passing through the gel pores
during the supercritical drying [144,145].
Instead, this class of polymer composite aerogels seems to be
straightforward to make mainly xerogel composites through the introduction of the monomer inside of the initial sol or by polymerizing of
the appropriate monomer in the post-gelation steps. The composites obtained from this method show a decrease in the elastic modulus with
substantial increase (3 times) in their ultimate compressive strength
[144].
From a practical point of view, the ambient dried aerogel-like class I
composites are advantageous, since they are cost effective and easy to
produce. Moreover, the elimination of the post-gelation processing
steps is an asset for further mass production of this type of materials
[157].
4.2.2. Class II hybrid composite aerogels
Class II hybrid aerogel composites are referred to the composite network in which the interfacial bonding between the organic phase and
silica is based on strong covalent bonds. This type of hybrid aerogels
was studied by Mackenzie et al. [158] who considered the different rearrangement of covalent bonding between the two phases.
Generally, this approach requires molecular precursors that contain
a hydrolytically stable chemical bond between the element that will
form the inorganic network during the sol–gel processing and the organic moieties. Since, in these materials, the polymer connects points
along the skeletal framework of 3D assemblies of nanoparticles, the
resulting composites are referred to as polymer cross-linked aerogels.
These materials possess the advantages of both organic and inorganic
materials and are expected to present unique properties that are different from the individual organic or inorganic materials.
H. Maleki et al. / Journal of Non-Crystalline Solids 385 (2014) 55–74
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Fig. 8. Organo(alkoxy) and bridged silanes used as precursors for sol–gel-derived (hybrid) materials.
Until now, several polymeric systems such as polyurea, polyurethane, poly (methyl methacrylate), polyacrylonitrile and polystyrene
have been used to reinforce silica aerogels [22–24,26,159]. In fact, the
underlying inorganic framework plays the role of structure-directing
agent (template). The improvement of mechanical properties is attributed to reinforcement of interparticle necks, which are the weak points
of the aerogel skeletal network (Fig. 1). In turn, the stabilization provided by the cross-linking polymer is attributed to the extra chemical
bonds created by interparticle polymeric tethers.
Extending polymer cross-linked aerogel composition to additional
polymers, optimization processes for desired material properties and
simple manufacturing are currently the focus of attention of several
research groups such as Leventis et al. [26,119], and the group led by
Meador [29,32] in the NASA Glenn Research Center.
4.2.2.1. Liquid and vapor phase polymer cross-linking. Native silica
aerogels can be reinforced with different organic polymers, starting
from the functionalization of secondary silica particle surfaces with appropriate functional groups. The functionalization of the surface of silica
particles within the inorganic network can be achieved through cocondensation of the core precursors, namely TMOS or TEOS, with silica
precursors containing the special organic functionality in their chemical
structures. Then, the polymerization normally can be performed by introducing the organic monomer through the post-gelation solvent
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Fig. 9. a) Stress strain curves of a monolithic sample of aerogel made from TMOS and a high portion of BTSPD; b) recovery after compression vs. time. The inset is showing the aerogel
sample compressed by finger pressure and demonstrating full recovery. Reprinted from Ref. [21], with permission from the Royal Society of Chemistry (RSC).
exchange and then promoting the reaction of the silica functionality
with monomers (Fig. 12). This multistep liquid-phase based reinforcing
approach is quite long due to the solvent exchange process, the slow diffusion rates of monomers in the wet gel and the heating necessary to
promote the reaction of the monomers with the silica surface, which
is followed by more solvent exchanges and supercritical drying
[23,28,29,48,52,53,84,160–162]. In addition, the monomer is usually introduced in the wet gel by soaking the gel inside of the monomer solution and, thus, sometimes the monomer does not uniformly diffuse in
the gels and, during the replication process, deviations of the aerogel
properties are detected [49]. In order to eliminate these time consuming
and tedious post-gelation steps, two effective alternatives to the previous approach can be used. The first alternative is to introduce the organic monomers in the initial sol, which contains the whole silica
precursors and gelation solvent, in order to carry out the procedure in
a simple one pot fashion [22,49,163]. This method provides the in situ
formation of inorganic silica network with organic monomer contained
in the pores. As in this step the monomers are already present inside the
inorganic matrix of the wet gels, the diffusion of the monomer to the gel
is not a limiting factor, therefore, the polymerization reaction is more
Fig. 10. Three dimensional network of MTMS/BTMSH-derived silica aerogels with its detailed molecular structure.
H. Maleki et al. / Journal of Non-Crystalline Solids 385 (2014) 55–74
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Fig. 11. Reaction strategy for preparation of non-modified polysilsesquioxane aerogels with incorporation of four types of flexible linking groups in the silica backbone. The chemical structures of the alkyl linked bis-silane precursors are indicated in Fig. 8 with their related numbers.
efficient. Moreover, the elimination of time consuming steps of soaking
the wet gel inside the monomer solution, in the in situ process, makes it
more cost effective [54]. Fig. 13 presents precisely the difference between the two liquid based routes for the development of polymer reinforced silica aerogels. The other recent approach for eliminating the
post-gelation treatment is to cross-link the aerogels through gas phase
after they have been supercritically dried [25,164]. This can be carried
out through the deposition of a desired monomer throughout the
pores' surface of an aerogel, by means of chemical vapor deposition
(CVD) or atomic layer deposition (ALD) [25,164–166].
Overall, all liquid-phase cross-linking strategies, whether one pot or
multi step, cause significant increase in strength (up to 8 MPa) but, on
the other hand, they increase the density (≈0.5–0.8 g.cm−3) and
decrease the surface area (≈40–600 m2.g−1). The density and then mechanical properties can be well controlled by varying several factors,
such as silicon molar concentration in the sol [21,23,29,48,49,138,167],
the organic monomer concentration [23,48,49,51,138,163], the gelation
solvents [138,167] and so on. The best samples with optimum physical
and mechanical properties were selected by some authors through
a statistical DoE studies [29,32].
For the CVD procedures, the density of the silica aerogel composites was in the range 0.095–0.230 g.cm− 3, and it also could be controlled by variation of the exposure times of the functionalized
silica aerogel in the monomer vapor [25]. In this approach, the typical cross-linked aerogels can be 31 times stronger than the original
silica aerogels before CVD treatment, with only two-fold reduction
in the surface area.
Different possible polymer reinforcements of silica aerogels, with
their detailed chemical approaches, are extensively studied and
reviewed within different research groups. Table 1 summarizes some
examples of polymeric systems that have been applied to improve the
mechanical properties of silica aerogels by cross-linking. Some of the
physical and mechanical properties of the obtained materials are also
presented.
4.2.2.2. Advanced methods for polymer reinforcing of silica aerogels. As
outlined in the previous section, the one pot, in situ, cross-linking process of silica is efficient for large scale production of these materials
and can eliminate the time consuming of post-gelation treatments of
the wet gels. But, performing the cross-linking part with hydrolysis
and condensation of silica precursors is not always responsive to some
kind of polymerization techniques. In fact, gelation of silica is based on
acidic and basic catalyzed ionic processes, and to perform the crosslinking reaction simultaneously it is necessary to have a polymerization
system with higher activation barrier than gelation, or performing
through non-ionic process like free radical polymerization [119]. The
implementation of this procedure for free radical polymerization
seems to be effective but still needs further post-gelation thermal or
light treatment.
The free radical polymerization can be achieved through modification of silica surface with olefins. From there, the polymerization
follows the “grafting to” or radical coupling process, in which the
coupling of the radicals to the surface of silica aerogels occurs [52].
It can be also performed via surface decoration of silica with radical
initiator, followed by polymerization through the “grafting from” approach, initiating the polymerization from the surface of silica
aerogels [161,168]. With the modification of the silica surface with
Si-AIBN radical initiator (Fig. 14), Leventis et al. [161] applied the
“surface initiated grafting from” approach for integration of
polymethylmethacrylate (PMMA), polydivinylbenzene (PDVB) and
polystyrene (PS) in the silica network, with polymerization starting
from the surface of the silica aerogel. The radicals were used to initiate the polymerization of vinyl groups grafted to the surface of the
silica gel. Although this approach is significantly simpler and allows
highest control on polymer coating around silica, and mechanical
properties comparable with the “grafting to” approach, the polymerization still is uncontrolled, which increases the polydispersity of the
polymer. On the other hand, if a significant portion of the initiator remains in the silica matrix, it causes the decomposition of the azo
bridged silsesquioxane during the heating of the gels, and this
leads to a weakening of the mechanical properties.
In order to overcome the problem arising from free radical surface
initiated reaction, and to allow a greater control on molecular weight
of the polymer and, indirectly, on the mechanical properties of the silica
aerogels, Boday et al. [24] applied a controlled polymerization reaction
for cross-linking the surface of silica with PMMA. For this purpose,
they implemented the “’Atom Transfer Radical Polymerization”’
(ATRP) technique [169], for controlling the molecular weight and polydispersity of the polymer on silica aerogel surfaces. In their approach,
initially, the sol–gel processable ATRP initiator was synthesized. Afterwards, as shown in Fig. 15, the surface of silica was modified with the
ATRP precursor through co-gelation with TMOS precursor. Then, in
next step, the vinyl polymer grew from colloidal secondary particles of
silica via the ATRP process and, after supercritical drying; the polymer
covers the surface of the silica network and reinforced it. This approach
allows the incorporation of well-defined polymers with a versatile
methodology for polymerization with controlled amount of vinyl
groups and cross-linkable monomers. The resulted polymer reinforced
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Fig. 12. a) Concept of polymer reinforcement using reactive groups on the silica surface; b) Epoxy cross-linking of silica aerogels. Inspired in [49].
aerogels comprised polymers with polydispersities from 1.2 to 1.8. The
aerogel bulk densities were in the range of 0.177–0.47 g.cm−3, and flexural strength ranges from15.4 to 63.5 kPa, with surface areas in the interval 144–681 m2.g−1.
For the production of aerogels with approximately the same physical properties and mechanical strength by different reinforcing strategies, Table A.1 was built (see the Appendix A) to give a global
comparative idea about the cost efficiency to produce a typical cylindrical strong aerogel with 12 cm length, 1 cm of diameter and bulk density
of ≈0.3 g.cm−3. In this table, several issues such as consumption of
starting material, amount of solvent, drying conditions and resources
(representing mainly the energy needs) and time of preparation
(representing the human resources) are compared for different strategies considered in Section 4.2.
4.3. Fiber reinforced silica aerogels
One of the interesting methods for strengthening the silica aerogels
is to use fibers as a second phase. Incorporation of any commercially
available or laboratory developed fibrous supporting materials, such
as carbon nanofibers, fiberglass, insulation fibers, alumina tiles, dacron,
cotton wool and polymeric nanofiber, into the aerogel systems, is quite
effective in increasing the mechanical properties of aerogels. So far,
composites of fibers and silica aerogels have been produced by various
methods in order to fortify the structure of silica aerogels and to overcome the network poor mechanical properties. The fiber matrix will
support the aerogel and decrease the bulk size of aerogel within the
aerogel-fiber matrix composite. By introducing a sol into a fibrous batting network, allowing it to gel and performing drying by supercritical
H. Maleki et al. / Journal of Non-Crystalline Solids 385 (2014) 55–74
67
Fig. 13. a) Traditional method to prepare cross-linked aerogels. Each wash step takes 24 h, and heating to cross-link can take as long as 72 h (This process require approximately 1 L of
solvent to prepare one ~20 mL cylindrical monolith), b) One-pot synthesis of cross-linked aerogels; the gray counterparts are prepolymers in the sol that are inert to gelation. Once the
gel is formed, it can react to initiate cross-linking. After one wash, which removes any unreacted components, the monolith is supercritically dried.
Table 1
Examples of polymeric systems, with the method of cross-linking to silica aerogels and selected properties of cross-linked aerogels.
a
The rupture strength increased (32×) to 17.6 N.
Three-point bend-beam flexural strength.
Only in 25% strain.
d
Not reported.
b
c
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Fig. 14. Chemical structure of silica wet gels obtained from a bis-triethoxysilane derivative
of AIBN. Inspired in [161].
fluids extraction, fiber-aerogel composites can be produced. Fig. 16
shows the general process for fiber reinforcement of silica aerogels
with a typical scanning electron microscopy (SEM) micrograph of the
final silica aerogel-fiber composites.
Aerogel blankets have been produced with improved thermal and
mechanical performances by scientists at Aspen Aerogels Inc. [170].
This type of aerogel composite was made by pouring the silica precursor
solution on top of a fiber batting blanket, previously placed in a casting
container, and allowing gelation to occur within a few minutes after the
addition of a catalyst.
Zhang et al. have incorporated non-woven polypropylene fibers into
silica aerogels by immersing them into a silica sol [57]. The surface of
the resulting wet gel composites was modified with TMCS and hexamethyldisiloxane, in order to make it hydrophobic. This surface
modified composite gel was dried at ambient pressure and 200°C to
produce a hydrophobic aerogel-like silica-fiber composite. In this procedure, the authors claimed that fibers acted as a supporting skeleton
that could increase the mechanical properties of the silica aerogels, although no mechanical characterization was reported for these aerogel
composites.
Li et al. [55] developed a flexible thin film of aerogel, reinforced with
incorporation of electrospun polyurethane (PU) and containing
poly(dimethylsiloxane) (PDMS) in the initial sol. In their procedures,
by controlling the kinetic of the sol–gel reaction, the polymer nanofibers (previously developed by electrospinning) were immersed inside
of a low viscosity sol before the gelation process. In this method, the fragility of the developed thin film aerogel composites was reduced by
strengthening the mesoporous structure in three dimensions in different scales: i) macroscopically, through electrospun fibers; ii) microscopically, by thickening the interconnecting interparticle necks within
the mesoporous silica with a silanol terminated poly(dimethylsiloxane)
(PDMS) oligomer. With this procedure, bendable aerogels with high surface area (604–759 m2.g−1), large pore volume and maximum flexural
strength of 200 kPa were developed.
Karout et al. [58] studied biocatalysts based on carbon and ceramic
fiber composite-silica aerogels and examined the influence of fiber
reinforcement on biocatalytic activity of these aerogels for lipase encapsulation. The synthesized aerogels were very elastic, and could easily recover after five tests without significant loss of mechanical integrity.
Paramenter et al. [171] have reinforced silica aerogels with 5 to
25 wt.% of ceramic fibers of silica, alumina and aluminoborosilicate
with different diameter and length. During supercritical drying, a high
proportion of fibers supports the matrix, reduces the shrinkage of silica
aerogels and results in a lower matrix density. In this method, the hardness and compressive strength did not show any regular trends with
fiber percentage for a given matrix density. However, the elastic moduli
increased with an increase in fiber percentage for a given matrix density
[171].
Fig. 15. Formation of an initiator-modified gel, ATRP growth of PMMA on surface and supercritical drying to afford a silica-PMMA composite aerogel. Adapted from Ref. [24].
H. Maleki et al. / Journal of Non-Crystalline Solids 385 (2014) 55–74
69
Fig. 16. General procedure for preparation of the fiber-silica composite aerogel. The SEM photo is related to a hydrophobic silica aerogel reinforced with polypropylene nonwoven fibers,
which was prepared by Zhang et al. Reprinted from Ref. [57], by permission of the publisher (Taylor & Francis Ltd, http://www.tandf.co.uk/journals).
Recently, Yuan et al. [126] reported the synthesis of silica aerogelfiber glass composites, using a press forming method. They studied the
effects of the addition of glass fiber and TiO2 as a second phase and the
forming pressure on improving the heat insulation and the mechanical
properties of the silica aerogel composites. It has been proved that by addition of the glass fibers, the strength of the composites was improved
but the heat insulation property was destroyed. They have shown that
for an increase of the forming pressure from 0.5MPa to 2.0MPa, the compressive strength of the composites increased from 0.3 MPa to 1.2 MPa
and the apparent density increased from 0.35 g.cm−3 to 0.53 g.cm−3.
Authors claimed that with an increase of the forming pressure, the precursor was pressed tightly and the fibers interweaved with each other,
and strengthened the aerogels.
Insoluble polysaccharides, such as cellulose and biologically based fibers, are other candidates for reinforcing of inorganic silica aerogels
[172,173]. Cai et al. [174] synthesized a high durable and flexible
cellulose-silica nanocomposite aerogel with in situ synthesis of silica
in cellulose gels. In their report, firstly the native cellulose was prepared
as a hydrogel with cellulose II crystallinity through dissolution and coagulation. The resulting gels displayed significant mechanical strength,
light transmittance and open structure. Then, the nanoporous gel structure of cellulose, with interconnected nanofibrillar network, was
impregnated with TEOS (in MeOH/H2O). The silica loading was more
than 60 wt.%. After synthesizing of the silica gel by sol–gel, the final
composite gel was dried with supercritical CO2 and resulted in the
cellulose-silica aerogel composites. Fig. 17 shows the typical mechanical
strength of this flexible cellulose-silica composite aerogel [174]. The
compression modulus of this composite is more than two orders of
magnitude higher than that of a silica aerogel, and about 50 times
higher than that of the aerogel prepared from bacterial cellulose [175].
In this method of reinforcement, in contrast to the increased density
from 0.14 g.cm−3 for native silica aerogels to about 0.6 g.cm−3 for composite aerogel, the thermal conductivity of the composites, which was
determined by the steady-heat-flow method, increased only from approximately 0.025 W m−1 K−1 to 0.045 W m−1 K−1. The synthesized
cellulose-silica composite can be useful as thermal insulator material
with high mechanical strength and stability, together with the ability
to form sheets, fibers, or beads.
Meador et al. [27] studied the use of the carbon nanofibers as reinforcement agents for polymer cross-linked aerogels. They examined
the effect of incorporation of 5 wt.% carbon nanofibers in di-isocyanate
cross-linked silica aerogels. Using statistical design of experiment,
they examined the effect of the addition of carbon nanofibers on mechanical and other properties of the resulting aerogels. According to
Fig. 17. a) Demonstration of the flexibility of cellulose-silica composite aerogel. b) Stress-strain curves of only cellulose aerogel (curve 1: tension, curve 3: compression) and cellulose-silica
composite aerogel (curve 2: tension, curve 4: compression). Reprinted from Ref. [174], by permission of the publisher (John Willy and Sons).
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their established model, it is possible to obtain a three-fold increase in
compressive modulus without any increase in density of the monoliths
by increasing the fiber concentration up to 5%. And also, a five-fold increase of tensile stress at break was obtained by incorporation of 5%
fiber, at lowest concentration values of total silane and cross-linker
agents. The density or porosity was not influenced with the addition
of the fibers, while the surface area was slightly reduced with increasing
of fiber concentration. Therefore, the incorporation of carbon nanofibers
provides a way to significantly improve the mechanical properties of
aerogels with densities bellow what can be achieved by cross-linking
with polymers. This is important for increasing the mechanical integrity
when a specific application requires high strength monoliths with high
porosity and low densities. As the author claimed, the biggest impact of
carbon nanofiber reinforced aerogels may be an improvement of
strength of hydrogels before polymer cross-linking, which results in
no breakage of fiber contacting wet gel during the initial demolding
process. So, this may allow the manufacturing of low-density polymer
reinforced aerogel at large scale.
Overall, in terms of fiber reinforcing of silica aerogels, it is necessary
to consider the chemical compatibility of both phases in terms of hydrophobicity and hydrophilicity [59]. In most cases, introducing chemical
covalent bonds between the fiber and silica network versus forming
only weakly physical bonds, may lead to long-term network integrity
and stability. Apart from the chemistry, different fiber matrices can require different processing conditions, which, in turn, originate different
reinforcement results. For instance, the methodology of reinforcing of silica aerogels with fiber blankets seems to be more straightforward and effective when compared to the reinforcement of aerogels with separate
and non-woven fibers [176]. In terms of non-woven fiber application,
the process is more tedious and complicated when compared to a fiber
blanket. That is due to the need of avoiding the sedimentation of the fibers, which also may lead in some extent to the inhibition of the silica gelation [55]. In this case, special care should be taken in the control of the
kinetic of the sol–gel reaction, namely the immersing time of the fibers in
the solution [55]. On the contrary, the reinforcing through a fiber blanket
can be provided simply by immersing the fiber blanket inside of the sol
before gelation, facilitating large scale production [176,177]. Overall,
the reinforcing of silica aerogels with fiber networks is more costly
than reinforcement of silica aerogels with polymers, since the starting
materials and organic monomers are normally cheaper than the fibers.
5. Outlook and future prospects
Tailoring the underlying silica backbone structures with more flexible
silica precursors, compounding the three-dimensional network of silica
structure with different polymeric systems and integrating a low percentage of different fibers in the silica network, improves not only the
strength of the aerogels, but also, in most cases, their elasticity [32]. In
general, increasing the amount of polymer cross-linker leads to a dense
and highly stiff structure of silica and weakens the thermal insulation
properties [161]. Thus, in this case, optimized polymer concentrations,
in combination with alterations made in the silica backbone structure,
should increase flexibility and strength with only slight differences in
other properties like density and thermal insulation properties. In this
sense, implementing one pot surface initiation polymerization from the
surface of the secondary silica particles, or doing polymerization with
well controlled polymerization techniques can be effective to strengthen
the aerogels without significantly increasing in their density [24,49,161].
Thus, in the present report, we have elucidated firstly the synthesis
chemistry and processing and different drying techniques to obtain silica aerogels; secondly, we summarized different recent strategies for
fabricating silica aerogels with improved mechanical properties.
Mechanically strong and low density monolithic aerogels have
potential to be widely used in space and armor applications as thermal insulators. However, namely in the field of polymer reinforced
aerogels, additional work is still required to produce aerogels at
commercial scale with certain configurations such as flexible thin
sheets. In thin film shape, the polymer-reinforced aerogels with
flexible linking groups would probably be too fragile to stand on
their own shapes. The recent bendable and flexible thin sheet processed by fiber incorporation in silica aerogels is an efficient candidate for insulating system, and could provide the requirement of
folding or wrapping around different space assemblages that are
needed for thermal insulation [55]. One interesting recent research
is the development of smart silica aerogels. Synthesis of shape memory aerogels by means of a shape memory polymer as reinforcement
agent is a promising area of investigation [60]. For instance, a polyurethane block copolymer as a shape memory polymer can be used
as reinforcing agent for the silica aerogel network [60]. This polymeric system is able to change its shape in response to external
stimuli. So taking advantage of this smart behavior, the aerogels
can be stored in deformed state for transport aboard a spacecraft.
Acronyms
ATRP
AIBN
ALD
CVD
DoE
EVA
EDL
ORMOSIL
PDMS
PMMA
PDVB
PS
PU
SFD
SEM
TEOS
TMOS
TMCS
Atom Transfer Radical Polymerization
2,2′-Azobis(2-methylpropionitrile)
Atomic Layer Deposition
Chemical Vapor Deposition
Design of Experiments
Extra-Vehicular Activity
Entry, Descent, and Landing
Organically Modified Silica
Polydimethylsiloxane
Polymethylmethacrylate
Polydivinylbenzene
Polystyrene
Polyurethane
Supercritical Fluid Drying
Scanning Electron Microscopy
Tetraethylorthosilicate
Tetramethylorthosilicate
Trimethylchlorosilane
List of common organosilanes used in this article
Monofunctional organosilanes of the type RSiX3:
APTES
ETMS
MTMS
MTES
VTMS
VTES
Aminopropyltriethoxysilane
Ethyltrimethoxysilane
Methyltrimethoxysilane
Methyltriethoxysilane
Vinyltrimethoxysilane
Vinyltriethoxysilane
Difunctional organosilane of the type R2SiX2:
DMDES
Dimethyldiethoxysilane
Bridged alkyl-linked organosilanes of the type X3Si-R-SiX3:
BTSPD
Bis[3-(triethoxysilyl)propyl]disulphide
BTMSH 1,6-Bis(trimethoxy silyl)hexane
BTMSPA Bis[3-(trimethoxysilyl)propyl)]amine
Acknowledgments
The research leading to these results has received funding from the
European Union Seventh Framework Programme (FP7-MC-ITN) under
H. Maleki et al. / Journal of Non-Crystalline Solids 385 (2014) 55–74
71
Table A.1
Comparison of the required resources for preparation of reinforced silica aerogels/aerogel-like monoliths using different synthesis methodologies (for an aerogel with 12 cm length, 1 cm
diameter and bulk density ≈0.3 g.cm−3).
grant agreement No. 264710. The authors would like to thank the
Directorate-General for Science, Research and Development of the
European Commission for financial support of the research.
Appendix A
Table A.1 presents a comparison of the required resources for the
production of an aerogel or an aerogel-like material of silica using several polymer-reinforcing methodologies, considering a given size/shape
of the monolith and approximately the same bulk density. This table
may be useful to estimate the cost inherent to each of these methodologies and to choose a certain route according with the existent resources. It should also be noted that due to the non-consistency
between the extracted synthesis/processing/drying information in the
literature, the data presented in this table is defined for the production
of lab scale aerogels with the conditions that are optimized within the
authors' research group.
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